In the discovery of new drugs, potential drug candidates are generated by identifying chemical compounds with desirable properties. These compounds are sometimes referred to as “lead compounds”. Once a lead compound is discovered, variants of the lead compound can be created and evaluated as potential drug candidates.
In order to reduce the time associated with discovering useful drug candidates, high throughput screening (HTS) methods are replacing conventional lead compound identification methods. High throughput screening methods use libraries containing large numbers of potentially desirable compounds. The compounds in the library are numerous and may be made by combinatorial chemistry processes. In a HTS process, the compounds are screened in one or more assays to identify those library members (particular chemical species or subclasses) that display a desired characteristic activity. The compounds thus identified can serve as conventional “lead compounds” or they can be therapeutic.
Conventional HTS processes use multi-well plates having many wells. For example, a typical multi-well plate may have 96 wells. Each of the wells may contain a different liquid sample to be analyzed. Using a multi-well plate, a number of different liquid samples may be analyzed substantially simultaneously.
It is desirable to reduce the volume of the wells in a multi-well plate to increase the density of the wells on the plate. By doing so, more wells can be present on the plate and more reactions can be analyzed substantially simultaneously. Also, as the volumes of the wells are reduced, the liquid sample volumes are reduced. Reducing the liquid sample volumes reduces the amount of reagents needed in the HTS process. By reducing the amount of reagents used, the costs of the HTS process can be reduced. Also, liquid samples such as samples of biological fluids (e.g., blood) are not always easy to obtain. It is desirable to minimize the amount of sample in an assay in the event that little sample is available.
While it is desirable to increase the density of the wells in a multi-well plate, the density of the wells is limited by the presence of the rims on the wells. The rims could be removed to permit the sample zones to be closer together and thus increase the density of the sample zones. However, by removing the rims, no physical barrier would be present between adjacent sample zones. This increases the likelihood that liquid samples on adjacent sample zones could intermix and contaminate each other.
Also, reducing the liquid sample volumes can be problematic. Decreasing the size of assays to volumes smaller than 1 microliter substantially increases the surface-to-volume ratio. Increasing the surface-to-volume ratio increases the likelihood that analytes or capture agents in the liquid sample will be altered, thus affecting any analysis or reaction using the analyte or capture agents. For example, proteins in a liquid sample are prone to denature at liquid/solid and liquid/air interfaces. When a liquid sample containing proteins is formed into a droplet, the droplet can have a high surface area relative to the amount of proteins in the droplet. If the proteins in the liquid sample come into contact with the liquid/air interface, the proteins may denature and become inactive. Furthermore, when the surface-to-volume ratio of a liquid sample increases, the likelihood that the liquid sample will evaporate also increases. Liquids with submicroliter volumes tend to evaporate rapidly when in contact with air. For example, many submicroliter volumes of liquid can evaporate within seconds to a few minutes. This makes it difficult to analyze or process such liquids. In addition, if the liquid samples contain proteins, the evaporation of the liquid components of the liquid samples can adversely affect (e.g., denature) the proteins.
Chips having elevated sample zones solve many of the problems associated with the use of multi-well plated for HTS processes (see U.S. patent application Ser. No. 09/792,335, filed Feb. 23, 2001, entitled “Chips Having Elevated Sample Surfaces”).
The identification of library members in HTS requires a fast and efficient method of analysis. The paucity of efficient library compound identification techniques remains a serious limitation for routine use of HTS processes such as protein analysis. Mass spectrometry is one such technique that can potentially be used for various HTS processes such as protein analysis. Mass spectrometry combines high sensitivity, selectivity and specificity with speed of analysis. For example, a complete mass spectrum can be recorded on a microsecond timescale.
Thus, there is a need in the art to adapt highly sensitive mass spectrometry techniques to high throughput screening methodologies such as protein analysis. Embodiments of the invention address, for example, these and other problems.